This invention generally relates to methods and apparatus for detecting highly electrically conductive bodies; and more specifically, to methods and apparatus that may be effectively employed to detect such bodies immersed or embedded in a medium having a relatively, or very, low electrical conductivity such as sea water, fresh water, sand, glass and rubber.
Media such as sea water, fresh water, sand, glass and rubber, which have an electrical conductivity between about 10.sup.4 and 10.sup.-8 siemens/meter, are commonly referred to as lesser conducting media. Various prior art methods and apparatus are known for detecting bodies in such media; and typically these methods and apparatus are used to detect submarines in sea water, although the methods and apparatus may be used for other purposes, such as to detect weapons or mines buried in sand or earth, or to detect underground cables or wires.
These prior art systems may be broadly divided into two classes. The first class, referred to as passive magnetic anomaly detection (MAD) systems, generally rely on a passive detection of anomalous magnetic fields created by inherent ferromagnetic properties of the target. Detection is typically by means of a magnetic field receiving means, or magnetometer, which is towed from a ship or mounted on an airborne platform.
One major disadvantage of this type of passive detection system is a potentially high false alarm rate, which may be caused by background magnetic signals from sources other than the body of interest, such as normal variations in the earth's own magnetic field. Because of this, the receiving means of the detection system must be relatively close to the ferromagnetic body in order to discriminate accurately between the weak inherent magnetic signal from the body and background magnetic signals, and passive MAD systems do not work well in coastal or shallow areas where magnetic signals caused by geological gradients compete with target signals. Furthermore, aircraft maneuvers and certain geomagnetic pulsations also may interfere with the operation of a passive MAD system, especially at high latitudes,
Another important disadvantage of the passive MAD system is that the target must be ferromagnetic. Because of this, the passive MAD system is not able to easily detect submarines with titanium hulls.
A second class of detection systems generally rely on passively monitoring for acoustic emissions from a target body, or on detecting active acoustic signals that are generated by the detection system and reflected by the target body back into a receiver of the detection system. These systems are known generally as passive and active sonar systems, respectively.
A characteristic of active sonar systems is that they usually rely on sound waves transmitted at frequencies above 1 kilohertz. However, acoustic signals at these frequencies attenuate rapidly in a medium such as sea water, and thus the range at which active sonar systems can detect a target is quite limited. Furthermore, while the receiver of an active sonar system monitors for a reflected acoustic signal, a receiver located on the target body itself may detect and counter the acoustic signal generated by the active sonar system at a much greater range. Therefore, for equally sensitive receivers, the target body, typically a submarine, may detect the acoustic signal generated by the sonar system at a distance approximately twice the distance at which the sonar system can detect the submarine echo. Thus, the submarine may be able to detect the presence of the sonar system and take evasive action or other counter measures before being detected by the active sonar system.
In addition, sound waves can bend upwards and downwards in water as the result of temperature and pressure gradients in the water. As a result of this bending, there may be zones or regions, referred to as sound shadow zones, in the water in which a target cannot easily be detected by an active sonar system.
Passive sonar systems, by contrast, generally rely on receiving sound waves having frequencies less than 200 hertz, that are emitted by submarine propellers, machinery, and auxiliary systems. However, submarine sound reduction programs have resulted in a very significant reduction in the amount of sound produced by certain submarines, and long range surveillance of such quiet targets cannot easily be achieved by passive sonar systems.
Other systems are known in which the phase change of a transmitted electromagnetic signal that is reflected by a body and detected by a receiver, is used to determine or measure distances; and, for example, U.S. Pat. No. 4,403,857 discloses such a system. It is also well known in radar technology to use the frequency shift imparted to an electromagnetic signal by a moving body to determine the velocity of that body.
The systems that measure distance based on the phase shift of a reflected electromagnetic wave have heretofore been limited to measuring distances through relatively non-conducting mediums, such as the distance between the detection system and an object located on the surface of an ocean, on land or in the air. A number of obstacles have prevented the adaption of such systems to detecting conducting bodies immersed in a lesser conducting medium.
A primary obstacle to using electromagnetic waves for this purpose is the rapid attenuation of electromagnetic fields and waves in lesser conducting media. For instance, the amplitude or strength of a 1 megahertz electromagnetic wave traveling through sea water attenuates by a factor of 1/e, referred to as 1 neper of attenuation, every 0.25 meter. Similarly, a magnetic field passing through sea water and having a magnetic flux density or strength at its source of 6.times.10.sup.5 Tesla, is attenuated by the sea water to a flux density of less than 10.sup.-6 nanotesla in less than 12 meters of the sea water. Since, with equipment suitable for use in a practical target detection system, 6.times.10.sup.5 Tesla is about the maximum magnetic field strength that can be currently generated, and 10.sup.-6 nanotesla is about the minimum magnetic field strength that can be detected, such a system could use a magnetic field to detect a target in sea water only to a depth of about 6 meters. However, submarines typically operate in seawater at depths of as low as 450 to 500 meters, and thus submarines cannot be detected at such depths using a system as outlined above.
Another obstacle to the use of electromagnetic waves to determine distances in lesser conducting media is the rapid phase change of an electromagnetic wave traveling in such a medium. More specifically, once the phase shift of an electromagnetic wave that is transmitted to and reflected off the target body, becomes greater than 2.pi. that phase shift is an ambiguous indicator of the position of the target body. A 1 megahertz electromagnetic wave transmitted through sea water undergoes a phase change of 2.pi. approximately every 1.6 meters, and thus such a wave cannot be used to determine unambiguously the actual depth of a body located more than approximately 0.8 meters below the surface of a body of sea water.